The purpose of this program is to elucidate basic molecular mechanisms which relate to the macromolecular behavior of DNA: its repair, recombination and mutagenesis. The understanding of such mechanisms should improve our understanding of the basic cellular processes involved, the effects of specific DNA modifications involved in mutagenesis, carcinogenesis and therapy, and the role of DNA repair mechanisms in response to both spontaneous and induced DNA damage. The components of the program all focus on this common goal but cover a spectrum of individual experimental systems, some procaryotic, some eucaryotic in such a way as to exploit maximally the advantages of each system. We propose to study genetic recombination in E. coli and in mammalian cells. Homologous pairing and strand exchanges which are mediated by recA protein will be studied by in vitro enzymologic methods. Particular structural damage in DNA which predispose to such recombination will be investigated. E.coli genes other than recA which control genetic recombination will be studied by formal genetic analysis and subsequently by biochemical approaches. In mammalian cell, homologous recombination between nearby duplicated genes encoding the herpes simplex thymidine kinase will be analyzed by varying the in vivo arrangement and structures of the substrates for recombination. The enzymology of the uvr repair system in E. coli will be studied with purified enzymes and defined substrates. Shuttle vectors which replicate and can be damaged in mammalian cells will be analyzed in E. coli to study both the repair and mutagenic action of specific DNA damage introduced in mammalian cells.